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Space Topics:
Near Earth Objects
What Is a Near-Earth Object?
Meteoroids, Asteroids, Comets
Near-Earth objects (NEOs) are collectively comprised of meteoroids, asteroids and comets.
Meteoroids are rocky bodies about 50 meters across or smaller whose impact with Earth is likely to deliver meteorite samples to the surface, but produce no impact crater.
The smallest meteoroids, about the size of grains of sand, produce nothing more than a flash across the sky, the result of the meteor’s encounter with Earth's atmosphere.
Asteroids, or minor planets, are rocky bodies larger than 50 meters across.
The largest of these known within near-Earth space is 1036 Ganymed, which has an estimated diameter of 41 kilometers (about 25 miles).
The second largest is 433 Eros. This 23-kilometer (14-mile) asteroid was the target for the Near-Earth Asteroid Rendezvous (NEAR) mission in 2000.
Whether an object is a comet or an asteroid is distinguished on the basis of the object's appearance through a telescope. If a newly discovered object appears fuzzy, then it is a comet. The "outgassing" that gives a comet its distinctive appearance is the result of ices vaporizing from the object's surface. It is likely that, for many comets, the surface ices evaporate completely after dozens of orbits around the Sun. If and when such extinct comets are found, their star-like appearance leads us to classify them as asteroids, even though they have a cometary origin.
At present, about 3,000 near-Earth objects have been discovered.
There are probably about 10,000 of these objects larger than 0.5 kilometers (0.3 miles) in diameter, suggesting that we have so far detected only a small percentage of their total population.
Comets are categorized according to their orbital period.
At present we only know of about 200 short-period comets -- a small fraction of their total population. Although most have orbits that carry them well above and below the plane of Earth's path, approximately 30 short-period comets have low enough inclinations to make them accessible from Earth with modest propulsion systems.
Objects in this category include Giacobini-Zinner, the first comet to be visited by a spacecraft (the International Cometary Explorer (ICE) in 1985), and Tempel 1, the target of the Deep Impact mission in 2005. The nuclei of these comets, which are about 10 kilometers (6 miles) across, may store virtually unaltered material from the earliest days of our solar system -- early protoplanetary nebula. And one of these materials -- water -- may prove to be a valuable resource for future human space explorers.
Maintaining the Supply of Near-Earth Objects
Whether they are asteroids or comets, NEOs are efficiently removed from the inner solar system by collisions or gravitational interactions with the terrestrial planets and the Sun.
Because this removal time is short compared with the age of the solar system, the NEO population we see today must be continually resupplied. (Any bodies that remained in the inner solar system immediately after the formation of the planets would have been depleted long ago.)
Thus, one or more processes must add new objects to the inner solar system at about the same rate with which they are removed by planetary encounters.
Comets appear to be supplied from both the Oort cloud and the Kuiper belt.
The Oort cloud is a hypothesized reservoir of primordial solar system material encircling the Sun more than 1,000 astronomical units (AU, the distance between Earth and the Sun) away.
The Kuiper belt is a disk of icy planetesimals residing between 30 and 1,000 AU away.
To date, scientists have discovered more than a thousand members of this belt.
According to one hypothesis, the fresh supply of near-Earth objects comes from the main belt of asteroids, the region between Mars and Jupiter where more than 100,000 objects have known orbits.
A process called chaotic dynamics may remove these objects from the asteroid belt and place them on trajectories that bring them near Earth. Suspect supply areas include Kirkwood gaps, highly depleted regions within the main belt that American astronomer Daniel Kirkwood first noted in 1867. The locations of these gaps correspond exactly to positions of “mean motion resonances” with Jupiter. At these locations, the orbital period of an asteroid is an exact integer multiple of Jupiter's. For example, at the 3 to 1 resonance, an asteroid completes exactly three revolutions in the time it takes Jupiter to complete one.
Danger from the Skies?
The hazard posed by NEOs is a matter of intense interest, especially since scientists now realize that the impact of an asteroid or comet was instrumental in the mass extinction (which included dinosaurs) at the end of the Cretaceous Period.
But one of the largest challenges in assessing the severity of the hazard is not knowing how many near-Earth objects there are.
By knowing the statistics for the population, we can gauge how frequently a devastating global impact occurs. Perhaps just as important will be an understanding of how frequently smaller collisions occur, such as the explosion that occurred over the Tunguska River of Siberia in 1908.
Surveys sponsored in part by The Planetary Society are just the beginning to achieve a census of the near-Earth object population. Larger surveys, such as the Spaceguard Survey, can reveal the full nature of the near-Earth environment. Using the information these surveys collect, we can make careful plans to protect Earth from future cosmic impacts.
source: http://www.planetary.org/explore/topics/near_earth_objects/facts.html
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I'm just wondering what navigating through these fields and clouds by ship, would entail in the further future , notwithstanding possible alternative technologies like wormholes.
I had this momentary nightmare vision of the asteroid belt, inhabited by space pirates, similar to the Gulf of Aden situation and the Somalian Pirates. (trapping cargo carriers and hijacking passenger cruisers - lol - it never ends.)
, but also figured that it might interfere with the planets given it's mass... didn't think about the asteroids though